It would depend on the size of the region, how built up it is, and also whether or not you intended to store your Plugins used for it.
My most built-up region at the moment is 30 large tiles in size (6 large tiles x 5 large tiles) and currently, the whole folder is 282MB. The largest, most built-up tile (with a population just shy of 400,000) is 37.1MB by itself, the least built up is 1.47MB, and the average (282MB divided by 30 city tiles) is 9.4MB. The mean of the smallest and largest is just under 19.3MB. To be safe, unless your region is really, really dense all the way across, 20MB per tile would figure to be a pretty safe figure.
Plugins folders vary considerably. The one I have in use for that region is about 4.4GB. I've heard of some people's folders exceeding 10GB. My full-blast NAM 37 install on its own, by the way, is a little under 1GB.
I'd say 16GB would handle pretty much any typical region/Plugins combination without issue. You might be able to get away with just 8GB or even 4GB if you're not dealing with anything large in either aspect.
-Alex